Local Electrode Potential Research Articles

Janus molybdenum di-chalcogenides based van der Waals bilayers for supercapacitor electrode design- effects of interlayer stacking orientations on quantum capacitance

In this work, for the first time, the quantum capacitance of an artificial two-dimensional (2D) material system based on van der Waals (vdW) bilayer heterostructures of Janus Molybdenum Di-chalcogenides, MoXY (X≠Y, X/YS, Se, Te) have been extensively investigated for electrode design of electrical double-layer (EDL) supercapacitor. The effects of different interlayer stacking orientations on local charge distribution, interlayer charge transfer, energy band structure, and density of states (DOS) have been comprehensively analyzed and correlated with the excess charge density and quantum capacitance variation with local electrode potential. Furthermore, the total capacitance reduction with respect to EDL capacitance has been analyzed over a wide range of EDL capacitance, and the performance of vdW Janus bilayers has been systematically benchmarked against the natural monolayers/bilayers and Janus monolayers of Molybdenum Di-chalcogenides. The results demonstrate that the large interlayer charge transfer induced built-in interlayer electric field significantly reduces the energy band gap of vdW Janus bilayers for S–Te and Se–Te interlayer stacking, thus leading to a significantly superior quantum capacitance. Finally, the favorable valley-dependent effective mass and band-degeneracy in MoSTe/MoSeTe have greatly enhanced quantum capacitance in S–Te and Se–Te stacking, rendering these bilayers a highly attractive candidate for supercapacitor electrode design.

Electrochemistry-Based Diagnosis of Li Plating for Fast-Charging Li-Ion Batteries

Recent innovations in battery technologies have facilitated the development and widespread use of electric vehicles (EVs), by increasing the energy density of batteries while lowering their cost. Despite the rapid growth of the EV market, the long charging time of lithium-ion batteries (LIBs) remains as a major obstacle for expanding customer acceptance of EVs. Typically, graphite-based anodes in the state-of-the-art LIBs suffer from Li plating under fast-charging conditions, which causes severe performance degradations and early cell failure of batteries. To prevent the Li plating-induced failure, extensive research efforts have been devoted to developing diagnostic methods capable of detecting Li plating. The electrochemical analysis of Li plating enables non-destructive, in operando detection without complicated experimental procedures or special equipment.Herein, we present a non-destructive method for detecting Li plating during fast-charge cycling by analyzing current transients during the constant-voltage charging step, based on electrochemical battery models. Three-dimensional (3D) and pseudo-two-dimensional (P2D) battery models are employed to predict the electrochemical behavior of full cells under fast-charging conditions. 3D model enables the evaluation of local electrochemical properties, such as local reaction current densities and electrode potentials, which are largely affected by the microstructure of the electrode. Furthermore, P2D models are employed to investigate the effect of changes in microstructural properties (e.g., porosity and fraction of active surface of the graphite anode) on electrochemical behavior of full cells.For 3D modeling, the 3D Voronoi tessellation and Catmull-Clark algorithm are employed to construct the model geometry of graphite anodes. 3D modeling results predict that upon fast-charging, the charge-transfer potential on the top surface of graphite anode rapidly drops below 0 V vs. Li/Li+, triggering Li plating. To extract the electrochemical signals of Li plating, the fast-charging behaviors of full cells are investigated by using the P2D model with a partially reversible Li plating–stripping reaction. In specific, the constant-current and constant-voltage (CC-CV) charging process of the full cells with graphite anodes is simulated with Li plating-induced degradation modes; reduction of (i) interparticle pores, (ii) active surface area for Li intercalation reaction, and (iii) cyclable Li amount. Modeling results demonstrate that an inflection point appears in the current transient curve, and it shows a positive shift when the Li plating-induced degradation occurs. Electrochemical tests and microstructural analyses further confirm the validity of proposed electrochemical detection of Li plating. This study provides an effective strategy to design electrochemical methods for Li plating detection during fast-charging batteries based on electrochemistry-based models.

Quantum capacitance engineering in boron and carbon modified monolayer phosphorene electrodes for supercapacitor application: A theoretical approach using ab-initio calculation

In this work, for the first time, the effects of Boron (B) and Carbon (C) substitutional doping/co-doping on the quantum capacitance of monolayer Phosphorene (Ph) have been extensively investigated for electrode design of electrical double-layer (EDL) supercapacitor. In this context, the effects of different doping concentrations on local charge distribution, energy band structure, and density of states have been systematically correlated with the quantum capacitance variation with local electrode potential. In pristine Ph, the presence of a large energy bandgap and the absence of states therein severely compromise the quantum capacitance at lower electrode potential compared to pristine Graphene (Gr). The B and C both act as acceptor-type impurities in Ph and tend to shift the Fermi level towards the valence band edge. Specifically, the introduction of B-doping induces localized defect states near the conduction band edges. This leads to a moderate improvement in quantum capacitance for a relatively larger electrode potential. In contrast, the C doping and B/C co-doping introduce de-localized defect states around the Fermi level near the valence band edges, which ensures a significantly larger quantum capacitance compared to pristine Gr and Ph over the range of −1.0 V to 1.0 V electrode potential.

Effect of Convection on Chirality of Spatiotemporal Patterns Observed in the Reduction of Hydrogen Peroxide

Background The spatiotemporal patterns have been reported in many electrochemical systems. Most of the patterns disappear when the reactions stop. This is because they originate from the spatial distribution of the local current and potential during the reactions. However, it is possible to inscribe the patterns into materials by utilizing the pattern formation in the material processing. This process enables to fabricate materials with unique shapes reflecting the pattern profile and is expected to be a new method for the production of such highly sophisticated materials. As one of the materials formed in this process, we have focused on the helical nanopore formed in the Platinum-assisted chemical etching of Si induced by the oxidation of Si and the reduction of H2O2. In this phenomenon, the helical pores are spontaneously formed by immersing the Si substrate modified with Pt nanoparticles into the etchant. We reported that the formation of helical pores originates from a spatiotemporal pattern emerging in the reduction reaction of H2O2 on the Pt particle. Our recent paper on helical nanopore formation in the etching of silicon implies that it is possible to fabricate chiral materials by utilizing the spatiotemporal pattern. In order to utilize the chiral material formed by the spatiotemporal pattern formation in the field of chiral chemistry, it is an inevitable task to control the chirality of the material. Because it is determined by the chirality of the pattern, we have to understand the effect of spatial symmetry breaking on the chirality of the spatiotemporal pattern. In this report, we focus on convection as the chiral condition introduced to the system and investigate its effect on the chirality of the spatiotemporal pattern. Among the several spatiotemporal patterns, we focus on the rotating waves (RW), where active and inactive sites rotate on the ring electrode because it is considered that this pattern emerges during the helical pore formation.Experimental In order to investigate the effect of the convection on the RW, we simulate the formation of the RW during the electrochemical reduction of H2O2 on the Platinum ring electrode based on the model constructed by Fukushima. In this model, we deal with the local electrode potential (E) and the local concentration of H2O2 (c) on the electrode surface as the variables. The differential equations governing E and c ware obtained by considering the equivalent circuit and the consumption and diffusion of H2O2, respectively. Since RWs have two different modes, which are the right-handed rotating wave (R-RW) and the left-handed rotating wave (L-RW), either may appear preferentially in the system where convection is considered. We conducted the simulation several times in a certain condition with random perturbation, and examined the percentage of R-RW in all RWs formed (R-RW%). In this simulation we always introduced the right-handed convection.Result and discussion The R-RW% changed depending on the velocity of the convection (v). When the convection was strong and v was 10 m/s, the R-RW was 0%, meaning that only the L-RW was formed. It should be emphasized that the RWs in the opposite direction to that of the convection, i.e. upstream RWs were formed in this condition. In contrast, when convection was mild and v was 0.1~1 m/s, the direction of the preferential RW was different from that of the RW formed when v was 10 m/s. In other words, the RW whose direction was the same as that of the convection (the downstream RW) was preferentially formed under mild convection. These results suggest that the formation of downstream RWs under mild convection is governed by the nonlinear dynamics. The observation of the change in the distribution of c and E showed that E in some area of the electrode got out of the potential region, where a negative differential resistance (NDR) appeared in the current-potential curve, at the initial stage of the RW formation. As a result, the double concavities emerged in the distribution of c. Therefore, we concluded that the competition of the effects of the convection on the double concavities and the whole concavity caused the change of the preferential RW with v.

Studies on Stress Corrosion Cracking of Vit 105 Bulk Metallic Glass

The project “Stress Corrosion Cracking of Zr-based Bulk Metallic Glasses” (SCC of Zr-BMG) within PP1594 mainly dealt with mechanical-corrosive interactions and failure of this class of metastable materials. It focussed on one of the most application-relevant Zr-BMG, Vit(reloy) 105 with composition Zr52.5Cu17.9Ni14.6Al10Ti5 (at.-%). Even though this BMG is known as extraordinary glass former, the metallurgical processing is still a critical issue. In contrast to conventional processing, i.e. melting of master alloys from pure elements, a different route using binary pre-alloys for the master alloys production was applied and led to superior mechanical properties upon mechanical testing under tensile and 3-point-bending (3PB) conditions in air. As a reference and for a detailed understanding of failure, fracture and cracking of Zr-base BMG in air, notched specimen 3PB experiments with in-situ microscopic observation were done and the still controversial interpretation of the mechanical behaviour of BMG in the framework of fracture mechanics was addressed. The specimen from the in-situ 3PB-tests served for TEM investigations on the structural nature of shear bands in BMG on the atomistic scale. Altogether, complete crack paths could be observed and analysed and based on this, details of the shear-band driven crack growth are described. While in first SCC studies using a newly developed setup full cross section (3PB) bars were investigated, in recent in situ experiments notched specimen were tested in 0.01 M NaCl yielding strong evidence for a catastrophic failure due to hydrogen embrittlement (HE). The known susceptibility to pitting corrosion in halide containing environments is only the initial stage for failure under SCC conditions. Once pitting is initiated, the local electrode potential is severely reduced. Further, the hydrolysis reaction of oxidised Zr4+ to zirconyl ions ZrO2+ during local BMG dissolution produces H+ and thus, a local acidic environment that enables proton reduction and hydrogen absorption in the stressed BMG region. The peculiar failure and fracture surface characteristics as well as the proven local reduction of the pH value in the vicinity of the notch during in-situ experiments clearly account for the proposed HE-SCC failure mechanism.

Impact of hydroxide ion–chloride ion concentration ratio on crack electrochemistry

ABSTRACT A model of the electrochemistry in a stress corrosion crack in martensitic stainless steel in low-chloride containing solution has been applied to predict the impact of the hydroxide ion–chloride ion ratio in the bulk solution on the local electrode potential and solution chemistry. Computations were undertaken for bulk chloride ion concentrations between 0.1 and 1000 ppm (by mass) and bulk pH values between 6.0 and 11.5. For near-neutral pH, a significant deviation of crack-tip electrochemistry from the bulk electrochemistry is predicted. However, above pH 9.5, a hydroxide-dominated regime is encountered with uniformity of potential, pH and chloride concentration.

First principles calculations on the influence of solute elements and chlorine adsorption on the anodic corrosion behavior of Mg (0001) surface

The influences of solute atoms (Li, Al, Mn, Zn, Fe, Ni, Cu, Y, Zr) and Cl adsorption on the anodic corrosion performance on Mg (0001) surface have been investigated based on first-principles calculations, which might be useful for the design of corrosion-resistant Mg alloys. Work function and local electrode potential shift are chosen as descriptors since they quantify the barrier for charge transfer and anodic stability. We found that at 25% surface doping rate, Y decreased the work function of Mg, while the impact of remaining doping elements on the work function of Mg was trivial due to the small surface dipole moment change. The adsorption of Cl destabilized the Mg atoms at surface by weakening the bonding between surface Mg atoms. We find that a stronger hybridization of d orbits of alloying elements (e.g. Zr) with the orbits of Mg can greatly increase the local electrode potential,which even overbalances the negative effect introduced by Cl adsorbates and hence improves the corrosion resistance of Mg alloys.

Environmental TEM Investigation of Electrochemical Stability of Perovskite and Ruddlesden–Popper Type Manganite Oxygen Evolution Catalysts

AbstractThe sluggish kinetics of the oxygen evolution reaction (OER) is a grand challenge for energy storage technologies. Several perovskites and other oxides of earth‐abundant elements are found to exhibit improved catalytic OER activity. However, less attention is paid to the electrochemical stability, an important factor for large‐scale application. The ongoing search for stable catalysts calls for characterizing active catalyst surfaces and identifying mechanisms of deactivation, activation, or repair. In situ techniques are indispensable for these tasks. This study uses environmental transmission electron microscopy on the highly correlated perovskite Pr1–xCaxMnO3 and the Ruddlesden–Popper Pr0.5Ca1.5MnO4 as model electrodes to elucidate the underlying mechanisms of the stability trends identified on rotating ring disk electrodes. An electron beam at fluxes well below those that would cause radiation damage is used to induce positive local electrode potentials due to secondary electron emission, driving electrochemical reactions in H2O vapor. The stability of the model systems increases with increasing ionic character of the MnO bond, while more covalent bonds are prone to corrosion, which is triggered by formation of point defects in the oxygen sublattice.

Quantitative Measurement of Local Fuel Cell Electrode Potentials

Measurement of local electrode potentials in operating fuel cells can provide important information about spatial variations in performance and durability. These measurements are useful for improved understanding of localized effects associated with contaminants, startup/shutdown, electrode irregularities, membrane cracks/holes, and fuel starvation. Accurate measurement of local electrode potentials requires incorporation of multiple reference electrodes into the fuel cell hardware. By connecting external reference electrodes to the outside of the MEA using ionomer salt bridges, local potential measurements can be obtained without disturbing cell operation [1]. Contaminants such as CO can cause strong localized electrode poisoning in operating fuel cells. Incorporation of multiple reference electrodes has enabled measurement of the distribution of anode losses in a fuel cell exposed to CO, as depicted in Fig. 1. Similar techniques can be used to measure and deconvolute losses associated with the anode and cathode. However, robust ionic connectivity between the MEA and the external reference electrodes is required to enable quantitative potential measurements. Excessive impedance in this ion-conducting pathway can result in slow response times, reference potential drift, and non-quantitative measurements. This work describes improved techniques for integrating external reference electrodes, allowing more accurate quantification of local electrode potentials. Fig. 1. Change in cell voltage and local anode potential (vs SHE) during exposure to CO gas (~400 s) and removal of CO (~1,800 s). References E. Brightman, G. Hinds, Journal of Power Sources 267 (2014) 160-170. Acknowledgements This research is sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and the U.K. National Measurement System. Figure 1

Measurement of Local Electrode Potentials in an Operating PEMFC Exposed to Contaminants

The use of a three-electrode cell, in which the potential of a working electrode can be accurately measured vs. a reference electrode (RE), is a standard practice in liquid-phase electrochemical experiments. However, the geometric constraints of a working PEMFC make use of a RE challenging. Incorporation of a RE into an operating PEMFC typically requires experimental compromises that prevent precise and accurate measurements from being obtained, and do not allow accurate spatial resolution of potential variation along the electrodes. As a result, PEMFC experiments are often performed using the PEMFC anode, which remains within a few mV of 0 V vs RHE during typical operation, as a pseudo RE. Recently, the National Physical Laboratory (NPL) developed an effective technique for accurate in situ monitoring of local electrode potentials in a PEMFC. By inserting REs from the back side of the MEA, the NPL configuration eliminates the edge effects, lack of anode vs. cathode specificity, Ohmic losses, and current density distribution disruptions that have compromised previous fuel cell RE experiments. We report here on the use of this RE technique to monitor local electrode potentials in an operating PEMFC during exposure to CO gas (Figure 1). CO is a common impurity in H2 gas and is known to poison Pt-based anodes. Levels of CO present in H2 fuel are expected to be in the ppb range for cells operating on pure H2 and the ppm range or higher for cells operating on reformate. Use of an RE can reveal the distribution of CO losses, and can also enable losses on the anode to be distinguished from those on the cathode. The results of this work demonstrate a new diagnostic capability that enables improved understanding of local conditions during PEMFC operation during transient or steady state operation. Figure 1. Cell voltage (orange) and anode potential (blue) of a MEA operating at 1 A/cm2 with anode exposed to 20 ppm CO. CO was removed from the anode feed at t = 47 min. Gore MEA, with anode/cathode loading of 0.4/0.1 mg/cm2, 80°C, 100% RH, 100 kPaabs. Figure 1

In situ mapping of potential transients during start-up and shut-down of a polymer electrolyte membrane fuel cell

Progression of a fuel/air front through the anode flow-field during start-up or shut-down of a polymer electrolyte membrane fuel cell is known to generate elevated cathode potentials, leading to corrosion of the carbon catalyst support. Here we present spatially resolved measurements of such potential transients in an operating fuel cell, using an innovative reference electrode array combined with quantification of carbon corrosion by measurement of CO2 in the cathode outlet. A systematic study of the effect of relative humidity on start-up/shut-down potential transients and carbon corrosion rates was carried out at open circuit and with the application of a small external load. The results are discussed in the context of a schematic framework for the reverse current decay mechanism expressed in terms of local electrode potential. In all cases carbon corrosion was more severe during start-up than during shut-down, with the highest cathode potentials measured opposite the anode outlet during start-up and opposite the anode inlet during shut-down. The carbon corrosion rate was least severe under the driest conditions, which was attributed to the increased membrane resistivity. This new technique provides a powerful diagnostic tool for evaluation of start-up/shut-down tolerant catalyst layers and optimisation of fuel cell hardware design.

Influence of corrosion in media with different pH on local electrode potentials of steels

We present results of investigations of the microelectrochemical heterogeneity of surfaces of steels 20, 30KhMA, and 12Kh21N5T after corrosion in chloride–sulfide media. The character of corrosion of steels in solutions containing chloride and sulfide ions is elucidated. It is shown that, during corrosion of steel 20 in solutions where passivation is observed, the microelectrochemical heterogeneity of the surface is higher than that after its active dissolution. Dense passivating films on the surface of the stainless steel cause its smaller microelectrochemical heterogeneity after corrosion in all investigated solutions than those for steels 20 and 30KhMA.

Analysis of different approaches to the study of microelectrochemical heterogeneity of materials

We investigated systems of steel 20 with nickel and zinc coating in a 3% NaCl solution, distilled water, and a 45 · 10 −3 % H 2 SO 4 + 0.14 H 2 O 2 + 5 · 10 −6 % K 2 Cr 2 O 7 solution using capillary probes filled with these environments. After their use, errors in measurements in the volume of the electrolyte are absent. The low conductance of the electrolyte and the dissolution of the metal in the active state are necessary conditions for adequate measurements of local electrode potentials. It is established that the accuracy of local microelectrochemical measurements in an electrolyte drop is higher than that in an electrolyte film.

Electrodeposition of Poly(N-vinylcarbazole) at the Three-Phase Junction. Formation of Very Different Polymer Structures

Poly(N-vinylcarbazole) films can be deposited at the three-phase boundary when the organic phase contains only monomer, N-vinylcarbazole, while the aqueous phase contains supporting electrolyte. A cylindrical platinum microelectrode is immersed into the two-liquid system in such a way that a part of it is located in one liquid and the other part resides in the second liquid. The thickness of the reaction layer, or the width of the microelectrode zone where the polymer grows, depends on the kind of ions present in the aqueous phase and the time of the experiment. The structure of the deposited polymer may be very different and depends on the distance from the three-phase boundary and the type of the anion present in the aqueous phase. The key parameters here are the local electrode potential and the local concentration of the anions. The list of obtained polymer structures includes, among others, arrays of perfectly arranged deep oval channels and groups of microcrystals. The porosity of the polymer deposits increases with the distance from the aqueous phase.

Interfacial electron transfer at TiO 2 nanostructured electrodes modified with capped gold nanoparticles: The photoelectrochemistry of water oxidation

We have studied the photoelectrochemical behavior of nanostructured anatase electrodes modified with gold nanoparticles stabilized either with citrate (Au cit) or with tetraoctylammonium bromide (Au TOAB). An increase both in photocurrent and in photopotential was observed in 0.05 M NaOH upon modification with Au TOAB. This behavior is attributed to an efficient hole consumption at the gold nanoparticles, which implies an effective hole transport to them from the TiO 2 nanostructure, probably facilitated by the capping agent. Once in the Au particles, the holes would promote an oxidation process (O 2 generation), which requires that the Au nanoparticles attain a high enough local electrode potential. This is possible as long as the Au TOAB nanoparticles may behave as nanocapacitors with a very low capacity (ca. 1 aF) and thus the accumulation of a few holes may induce important changes in their potential. In acidic medium or in the presence of 0.1 M CH 3OH no enhancement was detected. In the case of TiO 2 modified with Au cit, no increase of the photoelectrochemical response was observed in any case. It is suggested that a proper design of the metal/semiconductor nanojunctions may lead to an enhanced charge separation and to the eventual development of photoinduced metal electrocatalysis.

Stability of uniform electrode states in the presence of ohmic drop compensation

It is known that the compensation of the ohmic solution resistance by means of a Haber–Luggin capillary (i.e. a reference electrode (RE) that probes the solution potential close to the working electrode (WE)) causes a global feedback that might destabilize the uniform electrode state. In this paper we demonstrate that the cell resistance which is compensated when placing the reference electrode (RE) on the axis of the cell and close to a ring-shaped working electrode (WE) manifests itself in the evolution equation of the local electrode potential in exactly the same way an external series resistor of the same magnitude but negative impedance would do. This implies not only that the strength of the global feedback can easily be measured and thus its impact on the system’s behavior predicted but also that any external electronic cell compensation exerts the same destabilizing global coupling on the system, independent of relative arrangement and geometry of the electrodes. Evidence of this statement is given in three steps: First, a general formulation of the global coupling for the mentioned electrode geometry in terms of the cell resistance and the compensated resistance is derived. Then the spatio-temporal dynamics of electrochemical systems in which the cell resistance is compensated (a) by a close RE and (b) by an external negative impedance are compared in realistic three-dimensional simulations as well as in experiments.

Tip effects in the scanning-tunneling microscopy of semiconductor electrodes

The surface properties of a WSe 2 electrode have been investigated with a scanning-tunneling microscope in an electrochemical cell. Current–potential curves have been obtained for various electrode potentials and tunneling distances. They show a strong influence of the tip on the local electrode potential. The main features can be understood within a simple model describing the distribution of the electrostatic potential and its effect on the local electronic density.

Adaptation of the Scanning Vibrating Electrode Technique to ac Mode: Local Electrochemical Impedance Measurement

A novel method of measuring local electrochemical impedance using a single vibrating probe electrode has been developed by modification of a commercial device of scanning vibrating electrode technique (SVET). The small size of the vibrating probe used by the SVET, increases the space resolution of the current distribution. Furthermore, the local impedance as defined here takes into account not only the local current density but also the local electrode potential, as opposed to the measurements proposed up to now in the literature. Simulation calculations demonstrate that this definition is the only one which leads to understandable results. The signal processing used to obtain this impedance is explained in detail and tested on a dummy cell including a phototransistor to simulate a vibrating electrode. The local impedances of a corroding Ni disk and of an electrode simulating pitting corrosion are also measured with this system.

Synergistic effects of chromium depletion and ohmic potential drop on the susceptibility to intergranular corrosion and hydrogen embrittlement of sensitized stainless steel

This paper explores the consequences and analyzes the kinetics of the intergranular corrosion of the Cr-depleted zones in sensitized stainless steel. The corrosive attack of the Cr-depleted zones generates occluded grooves along the grain boundaries (g.b.) which are unable to passivate under the prevailing potential. The g.b. grooves deepen and branch beneath the surface until they eventually perforate the steel membrane. Furthermore, the resulting local electrode potentials within the g.b. grooves allow for hydrogen evolution even though the outer surfaces of the steel are held at much more oxidizing electrode potentials which prohibit this reaction. Using a Devanathan-Stachurski cell it was found that part of this ohmic ( IR)-produced hydrogen dissolves into the steel. The results are explained by the IR potential shift mechanism. Similarities and differences between this anodic charging and the conventional cathodic hydrogen charging experiments are also presented.

A Novel Way of Measuring Local Electrochemical Impedance Using A Single Vibrating Probe

A novel method of measuring local electrochemical impedance using a single vibrating probe electrode has been developed by modification of a commercial device of scanning vibrating electrode technique (SVET). The small size of the vibrating probe used by the SVET minimizes the screening effect of the probe on the distribution of current in the region of analysis. Furthermore, the local impedance as defined here takes into account not only the local current density but also the local electrode potential, as opposed to the measurements proposed up to now in the literature. The signal processing used to obtain this impedance is explained in detail and tested on a dummy cell including a phototransistor to simulate a vibrating electrode. The local impedance of an electrode simulating pitting corrosion is also measured with this system.